A guy on craigslist is offering an iPod touch 4th gen 64 gb for $115. The screen is completely ruined (cracked all over) and would need to be replaced. Moneywise, how low do you think I should try to get him to? I have never replaced any screen before of an iDevice. Would it be worth it to try? Is it very hard?

You need to be careful. I once bought a broken powerbook off ebay because I'd fully disassembled and reassembled that model dozens of times and figured there was nothing that could go wrong with it that I couldn't fix.

When I got it, half internal screws were missing and there wasn't a single part that wasn't broken, literally everything, the sound board, even the modem. Somebody had been doing repairs and put together a machine with all the dead parts he'd pulled. Even the housing was in worse shape than any I'd worked on.

No, no. I'm going to make sure of my assumptions by looking at it and using it some before I purchase it. Assuming until then that it is fully functional and the only problem is that it's screen is badly shattered, how low do you think I should try to get him to go. Again, is it very hard to replace a screen yourself?

The digitizer and LCD are fused together for the 4th gen iPod touch. You will need a heat gun to get the old screen off and a small screwdriver. The new screen should come with adhesive strips to stick the new one on.

It's not really hard. I've replaced a couple dozen at my shop. You do have to be careful of a couple very thin connectors, but it's very do-able. iFixit has a prett good guide. Feel free to PM if you need a hand.